The purpose of this project is to further identify, isolate and characterize an SV40 induced 55,000 MW cellular protein in embryonic cells. Primary cells prepared from midgestation mouse, rat and hamster embryos expressed the 55,000 dalton protein constitutively without SV40 infection. The 2D tryptic peptide map showed that this protein is similar to SV40 induced 55K protein and is conserved evolutionarily in both embryonic and in SV40 transformed cells. The embryo protein is a phosphoprotein. The amount of the protein in embryo was found to be half that of SB40 transformed cells and its presence correlated with the age of the embryo (in the case of mouse), maximum being at 12 day and minimum at 16 day. The amount of the protein decreased also with the tissue culture passage of embryo primary cells. The expression of the protein did not correlate with tumorigenic transformation in established mouse cells.